Biology exam 4 study guide
Biology exam 4 study guide Bio 104
Kutztown University of Pennsylvania
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Final Study Guide Section 9.4 the Cell Cycle and Cancer Cancer: a cellular growth disorder that occurs when cells divide uncontrollably. Most cancers are the result of accumulating mutations that ultimately cause a loss of control of the cell cycle. Cancers usually follow a common multistep progression: Benign: most cancers begin as this – abnormal cell growth this is not cancerous, and usually does not grow larger. o Additional mutations may occur, causing the abnormal cells to fail to respond to inhibiting signals that control the cell cycle. When this occurs the growth becomes malignant. Meaning it is cancerous and possess the ability to spread. Characteristics of Cancer Cells: Lack differentiation o They are not specialized and do not contribute to the functions of a tissue. o They look distinctly abnormal. o Cancer cells can enter the cell cycle an indefinite number of times – immortal nature. Abnormal nuclei o The nuclei of cancer cells are enlarged and may contain an abnormal number of chromosomes (extra copies may be present, duplicated portions, or deleted portions). Do not undergo apoptosis o Ordinarily, cells with damaged DNA undergo apoptosis, or programmed cell death. Form tumors o The abnormal cancer cells pile on top of one another and grow in multiple layers, forming a tumor. o During carcinogenesis, the most aggressive cell becomes the dominant cell of the tumor. Undergo metastasis and angiogenesis o New tumors distant from the primary tumor produce enzymes that they normally do not express, allowing tumor cells to invade underlying tissues. They then travel through the blood and lymph, to start tumors somewhere else in the body – this process is known as metastasis. o The formation of new blood vessels is required to bring nutrients and oxygen to support further growth of a tumor. Additional mutations occurring in tumor cells allow them to direct the growth of new blood vessels into the tumor in a process called angiogenesis. Origin of Cancer Normal growth and maintenance of body tissues depend on a balance between signals that promote and inhibit cell division. When this balance is upset, conditions such as cancer may occur. o Thus, cancer is caused by mutations affecting genes that directly or indirectly affect this balance. The following two types of genes are affected: Proto-oncogenes: code for proteins that promote the cell cycle and prevent apoptosis. o When mutations occur in proto-oncogenes, they become oncogenes (cancer causing genes). Oncogenes promote the cell cycle regardless of circumstances. Resulting in uncontrolled cell division. Tumor suppressor genes: code for proteins that inhibit the cell cycle and promote apoptosis. o A mutation in a tumor suppressor gene is much like brake failure in a car, when the mechanism that slows down and stops cell division does not function, the cell cycle accelerates and does not halt. Section 9.5 Prokaryotic Cell Division Cell division in single-celled organisms, such as prokaryotes (bacteria and archaea, lack a nucleus and other membranous organelles found in eukaryotes), produce two new individuals. This is asexual reproduction – the offspring are genetically identical to the parent. This processes occurs by duplication of a single chromosome and distributing a copy to each of the daughter cells. Unless a mutation has occurred, the daughter cells are identical to parent. The Prokaryotic Chromosome: Prokaryotes have a chromosome, which is composed of DNA and a limited number of proteins. o Organized differently than eukaryotic chromosomes, which have many more proteins. The bacterial chromosome appears as an electron-dense region called nucleoid, which is not enclosed by a membrane. Binary Fission: Prokaryotes reproduce asexually by binary fission, which produces two daughter cells that are identical to the original parent cell. Before division takes place the cell enlarges, and after DNA replication occurs, there are two chromosomes. The chromosomes attach to a special plasma membrane site and separate by an elongation of the cell that pulls them apart. o During this period, a new plasma membrane and cell wall develop and grow inward to divide the cell. Comparing Prokaryotes and Eukaryotes: Prokaryotes (bacteria and archaea), protists (many algae and protozoans), and some fungi (yeast) are single-celled. Cell division in single-celled organisms produces two new individuals. o This is a form of asexual reproduction because one parent has produced identical offspring. o The single chromosome consists largely of DNA with a few associated proteins. During binary fission, this chromosome duplicates, and each daughter cell receives one copy as the parent cell elongates, and a new cell wall and plasma membrane form between the daughter cells. No spindle is involved in binary fission. In single-celled eukaryotic organisms such as multicellular fungi (molds and mushrooms), plants, and animals, cell division is part of the growth process. It produces the multicellular form we recognize as the mature organism. Cell division is also important in multicellular forms for renewal and repair. o This is a form of asexual reproduction. In eukaryotic cells, the chromosomes are composed of DNA and many associated proteins. The histone proteins organize a chromosome, allowing it to extend as chromatin during interphase and to coil and condense just prior to mitosis. o Each species of multicellular eukaryotes has a characteristic number of chromosomes in the nuclei. o As a result of mitosis, each daughter cell receives the same number and kinds of chromosomes as the parent cell. The spindle, which appears during mitosis, is involved in distributing the daughter chromosomes to the daughter nuclei. o Cytokinesis, either by the formation of a cell plate (plant cells) or by following (animal cells), is the division of the cytoplasm. Section 10.1 Overview of Meiosis In sexual reproducing organisms, meiosis is the type of nuclear division that reduces the chromosome number from the diploid (2n) number to the haploid (n) number. The diploid (2n) number refers to the total number of chromosomes, which exists in two sets. The haploid (n) number of chromosomes is half the diploid number, or a single set of chromosomes. In humans meiosis reduces the diploid number of 46 chromosomes to the haploid number of 23 chromosomes. Gametes: reproductive cells (in animals, these are sperm and egg), usually have the haploid number of chromosomes. Sexual reproduction: haploid gametes, which are produced during meiosis, subsequently merge into a diploid cell called a zygote. In plants and animals, the zygote undergoes development to become an adult organism. Homologous Pairs of Chromosome: In diploid body cells, the chromosomes occur in pairs. A pictorial display of human chromosomes, called a karyotype, shows the chromosomes arranged according to pairs. Homologous chromosomes (homologues): look alike; they have the same length and centromere position. The DNA sequence for the gene on one homologue may differ from that of the other homologue. Alleles: alternate forms of a gene. The DNA sequences of alleles are highly similar, but they are different enough to produce alternative physical traits. To properly produce a haploid number of chromosomes in gametes, you first have to double the amount of DNA. DNA replication occurs in S stage of interphase. When duplicated, a chromosomes is composed of two identical parts called sister chromatids, each containing one DNA double helix molecule. The sister chromatids are held together at a common region called the centromere. One member of a homologous pair was inherited from the male parent, and the other was inherited from the female parent when the haploid sperm and egg fused together (this is why the zygote have paired chromosomes). Paternal chromosome: blue Maternal chromosome: red Meiosis is Reduction Division: The central purpose of meiosis is to reduce the chromosome number from 2n to n. Meiosis requires two nuclear divisions and produces four haploid daughter cells, each having one of each kind of chromosome. The process begins by replicating the chromosomes, then splitting the matched homologous pairs to go from 2n to n chromosomes during the first division. The second division reduces the amount of DNA in n chromosomes to an amount appropriate for each gamete. o Once DNA has been replicated and chromosomes become a pair, they may exchange genes, creating a genetic mixture different from the parent. o The second division is necessary because after the first division each chromosome still has a sister chromatid. The end result of meiosis is four gametes with n chromosomes. Meiosis 1 The homologous chromosomes come together and line up side by side, forming a synaptonemal complex. o The process is called synapsis and results in a bivalent – that is, two homologous chromosomes that stay in close association during the first two phases of meiosis 1. o Sometimes the term tetrad is used instead of bivalent – the bivalent contains four chromatids. Chromosomes may recombine or exchange genetic information during this association. Following synapsis, homologous pairs align at the metaphase plate, and then the members of each pair separate. This separation means that only one duplicated chromosome from each homologous pair reaches a daughter nucleolus, reducing the chromosome number from 2n to n. Meiosis 2 The sister chromatids separate, becoming daughter chromosomes that move to opposite poles. The chromosomes in each of the four daughter cells now contain only one DNA double helix molecule in the form of a haploid chromosome. The number of centromeres can be counted to verify that the parent cell has the diploid number of chromosomes. o At the end of meiosis 1, the chromosome number has been reduced, because there are half as many centromeres present. o At the end of meiosis 2, sister chromatids separate, and each daughter cell that forms still contains the haploid number of chromosomes, each consisting of a single chromatid. Fate of Daughter Cells In the plant life cycle, the daughter cells become haploid spores that germinate to become a haploid generation. o This generation will then produce gametes by mitosis. In the animal life cycle, the daughter cells become gametes, either sperm or eggs. The body cells of an animal normally contain the diploid number of chromosomes due to the fusion of sperm and egg during fertilization. Section 10.2 Genetic Variation Genetic variation is essential for a species to be able to evolve and adapt in a changing environment. Asexually reproducing organisms, such as prokaryotes, depend primarily on mutations to generate variation among offspring. In sexual reproducing organisms, the reshuffling of genetic material during sexual reproduction ensures that offspring will have a different combination of genes than their parents. Meiosis brings more genetic variation in two key ways: crossing over and independent assortment of homologous chromosomes. Genetic Recombination Crossing-over: is an exchange of genetic material between nonsister chromatids of a bivalent during meiosis 1. In humans it is estimated that an average of two to three crossovers occur between the nonsister chromatids during meiosis. At synapsis, homologues line up side by side, and a nucleoprotein lattice appears between them. This lattice holds the bivalent together so that the DNA of the duplicated chromosomes of each homologue pair is aligned. o This ensures that the genes contained on the nonsister chromatids are directly aligned. Now crossing-over may occur. As the lattice breaks down, homologues are temporarily held together by chiasmata, regions where the nonsister chromatids are attached due to DNA strand exchange and crossing-over. After exchange of genetic information between the nonsister chromatids, the homologues separate and are distributed to different daughter cells. Due to genetic recombination the offspring have a different set of alleles, and therefore genes, than their parents. This increases the genetic variation of the offspring. Independent Assortment of Homologous Chromosomes Independent assortment: the homologous chromosome pairs separate independently, or randomly. When homologues align at the metaphase plate, the maternal or paternal homologue may be oriented toward either pole. o 2^3 or 8 combinations of maternal and paternal chromosomes in the resulting gametes from this cell, simply due to independent assortment of homologues. Significance of Genetic Variation Fertilization: the union of the male and female gametes. (2^23)^2 different chromosome combination in the zygote in humans. This number assumes that there was no crossing-over between the nonsister chromatids prior to independent assortment. Asexual reproduction passes on exactly the same combination of chromosomes and genes. May be advantageous if the environment remains unchanged. If the environment changes, genetic variability among offspring introduced by sexual reproduction may be advantageous. Section 10.3 the Phases of Meiosis Meiosis consists of two unique, consecutive cell divisions, meiosis 1 and meiosis 2. DNA is replicated in the S phase of the cell cycle prior to meiosis 1 and not meiosis 2. Both meiosis 1 and 2 contain a prophase, metaphase, anaphase and telophase. Prophase 1 Spindle forms as the centrosomes migrate away from one another. Nuclear envelope fragments, and the nucleolus disappears. The homologous chromosomes (each consisting of two sister chromatids) undergo synapsis to from bivalents – and crossing over occurs. During prophase 1 the homologous chromosomes have condensed, now appearing as compacted metaphase chromosomes. Metaphase 1 The bivalents held together by chiasmata have moved toward the metaphase plate (equator of the spindle). Metaphase 1 is characterized by a fully formed spindle and alignment of the bivalents at the metaphase plate. Kinetochores are seen, but the two kinetochores of a duplicated chromosome are attached to the same kinetochore spindle fiber. Bivalents independently align themselves at the metaphase plate of the spindle. o Either the maternal or paternal homologue of each bivalent may be oriented toward either pole of the cell. Anaphase 1 The homologue of each bivalent separate and move to opposite poles, but sister chromatids do not separate. o This splitting of the homologous pair reduces the chromosome number from 2n to n. Telophase 1 The spindle disappears, but new nuclear envelopes need not form before the daughter cells proceed to meiosis 2. Interkinesis A short rest period prior to beginning the second nuclear division, meiosis 2. Similar to interphase – except that DNA replication does not occur, because the chromosomes are already duplicated. Meiosis 2 and Gamete Formation At the beginning of meiosis, the daughter cells contain the haploid number of chromosomes, or one chromosome from each homologous pair. The chromosomes align at the metaphase plate, but they do not align in homologous pairs, as in meiosis 1, because only one chromosome of each homologous pair is present. During anaphase 2, the sister chromatids separate, becoming daughter chromosomes that are not duplicated. These daughter chromosomes move toward the poles. At the end of telophase 2 and cytokinesis, there are four haploid cells. Following meiosis 2, the haploid cells become gametes in animals and in plants they become spores (reproductive cells that develop into new multicellular structures without the need to fuse with another reproductive cell). o The multicellular structure is the haploid generation, which produces gametes. o The resulting zygote develops into a diploid generation. o Therefore, plants have both haploid and diploid phases in their life. Section 10.4 Meiosis Compared to Mitosis In both meiosis and mitosis: An orderly serious of stages, including prophase, prometaphase, metaphase, and telophase are involved in the sorting and division of the chromosomes. The spindle fibers are active in sorting the chromosomes. Cytokinesis follows the end of the process to divide the cytoplasm between the daughter cells. Differences: Mitosis maintains the chromosome number between the cells. Meiosis is reduction division. Meiosis requires two nuclear divisions, but mitosis requires only one nuclear division. Meiosis produces four daughter nuclei. Following cytokinesis, there are four daughter cells. Mitosis followed by cytokinesis results in two daughter cells. Following meiosis, the four daughter cells are haploid and have half the chromosome number as the diploid parent cell. Following mitosis, the daughter cells have the same chromosome number as the parent cell. Following meiosis, the daughter cells are genetically identical neither to each other nor to the parent cell. Following mitosis, the daughter cells are genetically identical to each other and to the parent cell. Occurrence: Meiosis occurs only at certain times in the life cycle of sexually reproducing organisms. In humans, meiosis occurs only in the reproductive organs and produces the gametes. Mitosis is more common, because it occurs in all tissues during growth and repair. Meiosis 1 Compared to Mitosis: During prophase 1, bivalents form and crossing-over occurs – this does not happen in mitosis. During metaphase 1 of meiosis, bivalents independently align at the metaphase plate. The paired chromosomes have a total of four chromatids each. During metaphase in mitosis, individual chromosomes align at the metaphase plate. They each have two chromatids. During anaphase 1 of meiosis, homologues of each bivalent separate and duplicated chromosomes (with centromeres intact) move to opposite poles. During anaphase of mitosis, sister chromatids separate, becoming daughter chromosomes that move to opposite poles. Meiosis 2 Compared to Mitosis: In meiosis two the nuclei contain the haploid number of chromosomes. In mitosis, the original number of chromosomes is maintains. Meiosis two produces two daughter cells from each parent cell that completes meiosis 1, for a total of four daughter cells o The daughter cells contain the same number of chromosomes as they did at the end of meiosis 1. Section 10.5 the Cycle of Life Life cycle: refers to all the reproductive events that occur from one generation to the next similar generation. In animals and humans, the individual is always diploid, and meiosis produces the gametes, the only haploid phase of the life cycle. Plants have a haploid phase that alternates with a diploid phase. o The haploid generation is known as the gametophyte and the diploid generation is called the sporophyte. Gametogenesis: production of gametes. Spermatogenesis (males): occurs in the testes and produces sperm. o The testes contain stem cells called spermatogonia. These cells keep the testes supplied with primary spermatocytes that undergo spermatogenesis. Primary spermatocytes with 46 chromosomes undergo meiosis 1 to form two secondary spermatocytes, each with 23 duplicated chromosomes. Secondary spermatocytes undergo meiosis 2 to produce four spermatids with 23 daughter chromosomes. Spermatids then differentiate into viable sperm (spermatozoa). Oogenesis (females): occurs in the ovaries and produces eggs. o The ovaries contain stem cells, called oogonia that produce many primary oocytes with 46 chromosomes during fetal development. o The result of meiosis 1 is two haploid cells with 23 chromosomes each. o One of these cells, termed the secondary oocyte, receives almost all the cytoplasm. The other is a polar body (nonfunctioning, haploid cell) that may either disintegrate or divide again. o The secondary oocyte begins meiosis 2 but stops at metaphase 2. Then the secondary oocyte leaves the ovary and enters the uterine tube, where sperm may be present. If no sperm are in the uterine tube, or if a sperm does not enter the secondary oocyte, it eventually disintegrates without completing meiosis. If a sperm does enter the oocyte, some of its contents trigger the completion of meiosis 2 in the secondary oocyte, and another polar body forms. o At the end of oogenesis, following the entrance of a sperm, there is one egg and two or three polar bodies. The polar bodies dispose of chromosomes while retaining much of the cytoplasm in the egg. Cytoplasmic molecules and organelles are needed by a developing embryo following fertilization. A sperm and an egg join at fertilization, restoring the diploid chromosome number. The resulting zygote undergoes mitosis during development of the fetus. The mature egg has 23 chromosomes, but the zygote formed when the sperm and egg nuclei fuse has 46 chromosomes. Section 10.6 Changes in Chromosome Number and Structure Nondisjunction: a failure of chromosomes to separate – resulting in a gain or loss of chromosomes. Euploidy: the correct number of chromosomes in a species. Aneuploidy: a change in the chromosome number resulting from nondisjunction during meiosis. Seen in both plants and animals. Monosomy and trisomy are two aneuploidy states. Monosomy (2n -1): occurs when an individual has only one of a particular type of chromosome, when he or she should have two. Trisomy (2n + 1): occurs when an individual has three of a particular type of chromosome when he or should have two. Trisomy 21: down syndrome Primary nondisjunction: occurs during meiosis 1 when both members of a homologous pair go into the same daughter cell. Secondary nondisjunction: occurs during meiosis 2 when the sister chromatids fail to separate and both daughter chromosomes go into the same gamete. Karyotype: a visual display of the chromosomes arranged by size, shape, and banding pattern, may be performed to identify babies with Down syndrome and other aneuploid conditions. Changes in Sex Chromosome number An abnormal sex chromosome number is the result of inheriting too many or too few X or Y chromosomes. Extra copies of the sex chromosomes are much more easily tolerated in humans than are extra copies of autosomes. Turner syndrome: 1 copy of the X chromosome (female XO). Monosomy Klienfelter syndrome: more than one X and Y (male XXY). Trisomy Types of Chromosomal Mutations: Deletion: occurs when an end of a chromosome breaks off or when two simultaneous breaks lead to the loss of an internal segment. o William’s syndrome – deletion of chromosome 7. Duplication: the presence of a chromosomal segment more than once in the same chromosome. o Duplications may or may not cause visible abnormalities – depending on the size of the duplicated region. Inversion: when a segment of a chromosome has been turned around 180 degrees. o Most individuals with inversions exhibit no abnormalities, but this reversed sequence of genes can result in duplications or deletions being passed on to their children. Translocation: the movement of a chromosome segment from one chromosome to another, nonhomologous chromosome. o Alagille syndrome – translocation between chromosomes 2 and 20. Section 11.1 Gregor Mendel Genetics: explains the stability of inheritance as well as variations between offspring from one generation to the next. The Blending Concept of Inheritance : meant that a cross between plants with red flowers and plants with white flowers would yield only plants with pink flowers. When red and white flowers reappeared in future generations, the breeders mistakenly attributed this to instability in the genetic material. According to the blending concept, over time variation would decrease as individuals became more alike in their traits. Mendel’s Particulate Theory of Inheritance: called a particulate theory because it is based on the existence of minute particles, or hereditary units, we now call genes. Inheritance involves the reshuffling of the same genes from generation to generation. Mendel proposed two laws; the law of segregation and the law of independent assortment. The laws describe the behavior of these particulate units of heredity as they are passed from one generation to the next. Mendel was one of the first scientists to apply mathematics to biology. Mendel worked with the Garden Pea (Pisum sativum) Garden peas were easy to cultivate and had a short generation time. Garden peas could be cross-pollinated by hand by transferring pollen from the anther (male part of a flower) to the stigma (female part of a flower). Mendel chose 22 varieties of peas for his experiments. When these varieties self-pollinated, over generations they became true- breeding – meaning that all the offspring were the same and exactly like the parents plants. Mendel observed that the offspring did not possess intermediate characteristics but, rather, were similar in appearance to one of the parents. o This disproved the blending concept and supported the particulate theory of inheritance. Section 11.2 Mendel’s Laws After Mendel ensured all pea plants were true bleeding, he was ready to preform cross-pollination experiments. Monohybrid cross: mating between two individuals with different alleles at one genetic locus of interest. Law of Segregation: Each individual has two factors for each trait. The factors segregate (separate) during the formation of the gametes. Each gamete contains only one factor from each pair of factors. Fertilization gives each new individual two factors for each trait. Mendel’s Cross as Viewed by Modern Genetics The traits Mendel studied are controlled by single genes. These genes occur on a homologous pair of chromosomes at a particular location, called the gene locus. Alternative version of a gene are called alleles. A dominant allele will mask the expression of a recessive allele when they are together in the same organism. o The dominant allele codes for the protein associated with the normal function of the trait within the cell. o The recessive allele represents a loss of function, meaning it codes for a protein that has an altered function or no function within the cell. Homozygous: when an organism has two identical alleles. o Example: TT or tt Heterozygous: when an organism had two different alleles at a gene locus. o Example: Tt Genotype versus Phenotype Genotype: refers to the alleles an individual at fertilization. May be indicated by letters or by short, descriptive phrases and represents the DNA sequence for a particular gene. o Genotype TT is called homozygous dominant. o Genotype tt is called homozygous recessive. o Genotype Tt is called heterozygous. Phenotype: refers to the physical appearance of an individual, which is determined by the proteins produced by the corresponding alleles. The DNA that makes up the genotype produces the proteins that makes up the phenotype. Mendel’s Law of Independent Assortment Mendel performed a second series of crosses in which true-breeding pea plants differed in two traits. Dihybrid cross: mating experiment between two organisms that are identically hybrid for two traits. A hybrid organism is one that is heterozygous, which means that is carries two different alleles at a particular genetic position, or locus. The law of independent assortment states the following: Each pair of factor segregates (assorts) independently of the other pairs. All possible combinations of factors can occur in the gametes. Each chromosome carries a large number of alleles; however, the law of independent assortment applies only to alleles on different chromosomes. Mendel and the Laws of Probability Punnett Square: all possible type of sperm are lined up vertically and all possible type of eggs are lined up horizontally, and every possible combination of gametes occurs within the squares. Section 11.3 Mendelian Patterns of Inheritance and Human Disease Autosome: any chromosome other than a sex (X or Y) chromosome. 22 autosomal + 1 sex Autosomal Recessive Disorders Methemoglobinemia: lack of enzyme (diaphorase, coded for by a gene on chromosome 22) that converts methemglobin (picks up 02 to convert to CO2) to hemoglobin. o Victimized individuals are unable to clear the abnormal blue protein from their blood, causing their skin to appear bluish-purple. Cystic Fibrosis: faulty chloride channels in cell membranes causes mucus build up in the lungs. o Most common lethal genetic disease among Caucasians in the United States. Tay-Sachs: impairs function of an enzyme and leads to buildup of lipids. Autosomal Dominant Disorders Huntington Disease: neurological disorder that leads to progressive degeneration of brain cells. The disease is caused by a mutated copy of the gene for a protein called huntingtin. Section 11.4 beyond Mendelian Inheritance Multiple Allelic Traits When a trait is controlled by multiple alleles, the gene exists in several allelic forms within a population. A person’s ABO blood type is controlled by a single gene pair, three possible alleles within the human population determine blood type. Each person receives two of these alleles (one from each parent) to determine the presence or absence of antigens on his or her red blood cells. o IA = A antigen on red blood cells o IB = B antigen on red blood cells o i = Neither A nor B antigen on red blood cells The inheritance of the ABO blood group in humans is also an example of codominance, because both IA and IB are fully expressed in the presence of the other. Codominance: both alleles of a gene are equally expressed in a heterozygote. Incomplete Dominance: when a heterozygote has an intermediate phenotype between that of either homozygote. When both alleles are equally dominant as the other. Incomplete penetrance: a dominant allele may not always lead to the dominant phenotype in a heterozygote, even when the alleles show a true dominant/recessive relationship. The dominant allele in this case does not always determine the phenotype of the individual. Just because a person inherits a dominant allele doesn’t mean he or she will fully express the gene or show the dominant phenotype. Many dominant alleles exhibit varying degrees of penetrance. Pleiotropic Effects Pleiotropy: occurs when a single mutant gene affects two or more distinct and seemingly unrelated traits. Marfan syndrome MS: connective tissue mutation Sickle cell anemia: mutation in gene that affects hemoglobin molecule, affecting blood flow through capillaries and organs. Polygenic Inheritance: occurs when a trait is governed by two or more sets of alleles. Human height, skin color, and the prevalence of diabetes. Environmental Influences: Multifactorial Traits Multifactorial traits are those controlled by polygenes subject to environmental influences. Genetic disorders likely die to the combined action of many genes plus environmental influences. The relative importance of genetic and environmental influences on the phenotype can vary, and often it is a challenge to determine how much of the variation in the phenotype may be attributed to each factor. X-linked Inheritance X-linked: genes that have nothing to do with gender yet are carried on the X chromosome. The Y chromosome does not carry these genes and indeed carries very few genes. Morgan’s Experiment Male with white eyes x female with red eyes F1 X F1 (red-eyed x white-eyed) Morgan gathered that red eyes were the dominant characteristic and white were the recessive characteristic. F2 (red-eyed female, red-eyed: white eyed males) Carrier: a female who is heterozygous for an x-linked trait. Carriers do not always show a recessive abnormality, but they are capable of passing on a recessive allele for an abnormality. Hemophilia: person’s blood either does not clot or clots very slowly. Section 12.1 the Genetic Material Genetic material: early to mid-20 century research. When researchers began their work they knew that genetic material must be: 1. Able to store information that pertains to the development, structure and metabolic activities of the cell or organism. 2. Stable, so that it can be replicated with high accuracy during cell division and be transmitted from generation to generation. 3. Able to undergo rare changes, called mutations, that provide the genetic variability required for evolution to occur. Transformation of Bacteria: Fredrick Griffin was attempting to develop a vaccine against streptocococcus pneumonia (pneumococcus), which causes pneumonia in mammals. In 1931, he performed a classic experiment with the bacterium. Noticed that when these bacterium are grown on culture plates, some, called S strain bacteria, produce shiny, smooth colonies and others, called R strain bacteria, produce colonies that have a rough appearance. o S strain bacteria have a capsule (mucous coat) that makes them smooth, but R strain bacteria do not. o When Griffin injected mice with the S strain of bacteria, the mice died, but when he injected with R strain, the mice did not die. o When he injected the mice with heat-killed S strain bacteria, the mice did not die. o Finally, he injected the mice with a mixture of heat-killed S strain and live R strain bacteria. The mice died and living S strain was found the bodies. o Griffin concluded that some substances necessary for the bacteria to produce a capsule and be virulent must have passed from the dead S strain bacteria to the living R strain bacteria, so that the R strain bacteria were transformed. The change in the phenotype must have been due to change in the genotype. Later Oswald Avery determined the cause of transformation: DNA from S strain bacteria causes R strain bacteria to be transformed, so that they can produce a capsule and be virulent. The addiction of DNAse, an enzyme that digests DNA, prevents transformation from occurring. Molecule weight of transforming material is large. The addiction of enzymes that degrade proteins has no effect on the transforming substance, nor does RNAse, an enzyme that digests RNA. Thus, neither protein nor RNA is the genetic material. Alfred Hershey and Martha Chase studied viruses in the early 1950’s which helped to firmly establish DNA as the genetic material. Used radioactive isotopes to change protein. o To label DNA in bacteria. Viruses are made of DNA and a protein: inject material into cell and uses cell’s gen. machine to make copies of viruses’ genetic material. The Structure of DNA DNA contains four different types of nucleotides: two with purine bases, adenine (A) and guanine (G), which have a double ring; and two with pyrimidine bases, thymine (T) and cytosine (C), which have single ring. Erwin Chargaff used new chemical techniques developed in the 1940s to analyze in detail the base content of DNA. Chargaff’s rules: The amount of A, T, G and C in DNA varies from species to species. In each species, the amount of A = T and the amount of G = C. X-Ray Diffraction of DNA Rosalind Franklin: x-ray diffraction to look at DNA shape. Francis Crick and James Watson: franklin’s work told crick and Watson DNA was a helical figure (double-helix) molecule – with hydrogen bonded bases. DNA is a double helix with sugar-phosphate backbones on the outside and paired bases on the inside. DNA strands of the double helix are antiparallel, meaning that the sugar- phosphate groups that are chained together to make each strand are oriented in opposite directions. o Ensures that the bases are oriented properly, so that they can interact. Complementary base pairing means that a purine (large, two-ring base) is always bonded to a pyrimidine (smaller, one-ring base). o Hydrogen bonding between bases. The information stored within DNA must always be read in the 5′ to 3′ direction. Thus, a DNA strand is usually replicated in a 5′ to 3′ direction. Section 12.2 Replication of DNA DNA Replication: refers to the process of copying a DNA molecule. Following replication, there is usually an exact copy of the parental DNA double helix. A template is most often a mold used to produce a shape complementary to itself. During DNA replication, each DNA strand of the parental double helix serves as a template for a new strand in a daughter molecule. DNA replication is termed semiconservative replication, because each daughter DNA double helix contains an old strand from the parental DNA double helix and a new strand. DNA replication requires three main steps: unwinding, complementary base pairing, and joining. At the molecular level, several enzymes and proteins participate in the synthesis of the new DNA strands. Unwinding A DNA helicase enzyme unwinds DNA and separates the parental strands. This creates two replication forks that move away from each other. These separated strands now become the template to create two new DNA molecules. DNA is chemically stable as a helix, but not as single strands. Single-stranded binding proteins (SSB) attach to newly separated DNA and prevent it from re-forming the helix so replication can occur. Complementary Base Pairing DNA replication needs a primer, a short strand of RNA, to put in place before replication can begin. DNA primase places short primers on the strands to be replicated. DNA polymerase recognizes this RNA target and begins DNA synthesis, allowing new nucleotides to form complementary base pairs with the old strand and connecting the new nucleotides together in a chain. DNA polymerase also proofreads the strands and can correct any mistakes. The parental strands are antiparallel to each other, and each of the new daughter strands must also be antiparallel to its matching parental strand—which creates a problem. DNA can only be synthesized in a 5′ to 3′ direction. One strand, the leading strand, is exposed so that synthesis in a 5′ to 3′ direction is easier and replication is continuous. The other new strand in the fork must be synthesized in the opposite direction, requiring DNA polymerase to synthesize the new strand in short 5′ to 3′ segments with periodic starts and stops. This strand is called the lagging strand. Replication of the lagging strand is therefore made in segments called Okazaki fragments, after Japanese scientist Reiji Okazaki, who discovered them. Joining After both new strands are made, DNA polymerase has yet another role by converting the short RNA sequences, laid down by the primase, into DNA. Finally, the enzyme DNA ligase is the “glue” that mends all the Okazaki fragments together, resulting in the two double helix molecules that are identical to each other and to the original molecule. Prokaryotic vs. Eukaryotic Replication Prokaryotic DNA Replication: bacteria have a single circular loop chromosome. Eukaryotic DNA Replication: replication fork – the location where two parental DNA strands separate. Section 12.3 the Genetic Code First, the DNA undergoes transcription, a process by which an RNA molecule is produced based on a DNA template. DNA is transcribed, or copied base by base, into mRNA, tRNA, and rRNA. Occurs in the nucleus When mRNA is first made by RNA polymerase from the gene, it is in a rough form. Called pre-mRNA, it contains a mix of exons (protein-coding regions) and introns (non-protein-coding regions), particularly in multicellular eukaryotes. o Exons: Segment of mRNA containing the proteincoding portion of a gene that remains within the mRNA after splicing has occurred. o Introns: Intervening sequence found between exons in mRNA; removed by RNA processing before translation. Second, during translation, the mRNA transcript is read by a ribosome and converted into the sequence of amino acids in a polypeptide. Occurs in the cytoplasm The mRNA sequence determines the sequence of amino acids in a protein, it becomes necessary to identify the specific genetic code for each of the 20 amino acids found in proteins. 3 step process: o Initiation: a ribosome becomes assembled around the start codon. o Elongation: amino acids become bonded to form a polypeptide. o Termination: ribosome comes to a stop codon; mRNA and ribosomal subunits detach. Together, the flow of information from DNA to RNA to protein to trait is known as the central dogma of molecular biology. The Genetic Code: Triplet code: During gene expression, each sequence of three nucleotide bases stands for a particular amino acid. Codon: Three-base sequence in messenger RNA that during translation directs the addition of a particular amino acid into a protein or directs termination of the process. The genetic code is: degenerate unambiguous has start and stop signals Chapter 13: Regulation of Gene control Prokaryotic Regulation Operon: group of structural and regulating genes that function as a single unit. Regulator gene: normally located outside the operon, this codes for a DNA-binding protein that acts as a repressor. The repressor controls whether the operon is active or not. Promoter: a short sequence of DNA where RNA polymerase first attaches to begin transcription of the grouped genes. Basically, a promoter signals the start of the operon and the location where transcription begins. Operator: a short portion of DNA located before the structural genes. If a repressor is attached to the operator, then transcription cannot occur; conversely, if a repressor is not attached, then transcription can occur. In this way, the operator controls transcription of structural genes. Structural genes: these genes code for the enzymes and proteins that are involved in the metabolic pathway of the operon. The structural genes are transcribed as a unit. trp Operon: a) inactive repressor when tryptophan absent and b) active when tryptophan present – Feedback. Tryptophan: co-repressor lac operon: a) active repressor when lactose absent and b) inactive repressor when lactose present. Typical of catabolic reactions. Gene Mutations: Can alter the base sequence of DNA. Causes: o Spontaneous mutations result from normal biological processes. o Induced mutations result from exposure to toxic chemicals or radiation. Mutagens Effect of gene mutations on protein activity: o Point mutations o Sickle cell anemia o Frameshift mutation
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